Method for modifying a catalyst

ABSTRACT

A process for shape selective hydrocarbon conversion involves contacting a hydrocarbon feedsteam under conversion conditions with a modified catalytic molecular sieve which has been modified by being pre-selectivated with a first silicon source, then steamed. The feedstream may also contain a second silicon source which is a high efficiency para-xylene selectivating agent. The method for modifying the molecular sieve is also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 08/042,431,filed Apr. 5, 1993, now U.S. Pat. No. 5,349,144.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a shape selective hydrocarbonconversion process over a modified catalyst. The invention is alsodirected to the modified catalyst and method for modifying the catalystby pre-selectivating with first silicon source followed by steamtreatment.

2. Description of the Prior Art

The term shape-selective catalysis describes unexpected catalyticselectivities in zeolites. The principles behind shape selectivecatalysis have been reviewed extensively, e.g. by N. Y. Chen, W. E.Garwood and F. G. Dwyer, "Shape Selective Catalysis in IndustrialApplications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore,hydrocarbon conversion reactions such as paraffin isomerization, olefinskeletal or double bond isomerization, oligomerization and aromaticdisproportionation, alkylation or transalkylation reactions are governedby constraints imposed by the channel size. Reactant selectivity occurswhen a fraction of the feedstock is too large to enter the zeolite poresto react; while product selectivity occurs when some of the productscannot leave the zeolite channels. Product distributions can also bealtered by transition state selectivity in which certain reactionscannot occur because the reaction transition state is too large to formwithin the zeolite pores or cages. A final type of selectivity resultsfrom configurational diffusion where the dimensions of the moleculeapproach that of the zeolite pore system. A small change in dimensionsof the molecule or the zeolite pore can result in large diffusionchanges leading to different product distributions. This type of shapeselective catalysis is demonstrated, for example, in toluene selectivedisproportionation to p-xylene. The production of para-xylene istypically performed by methylation or disproportionation of toluene overa catalyst under conversion conditions. Examples are the reaction oftoluene with methanol as described by Chen et al., J. Amer. Chem. Sec.1979, 101, 6783, and toluene disproportionation, as described by Pinesin "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press,N.Y , 1981, p. 72. Such methods typically result in the production of amixture including para-xylene, ortho-xylene, and meta-xylene. Dependingupon the para-selectivity of the catalyst and the reaction conditions,different percentages of para-xylene are obtained. The yield, i.e., theamount of feedstock actually converted to xylene, is also affected bythe catalyst and the reaction conditions. The equilibrium reaction forthe conversion of toluene to xylene and benzene proceeds as follows:##STR1##

One method for increasing para-selectivity of zeolite catalysts is tomodify the catalyst by treatment with a "selectivating agents". Forexample, U.S. Pat. Nos. 4,950,835, 4,927,979, 4,465,886, 4,477,583,4,379,761, 4,145,315, 4,127,616, 4,100,215, 4,090,981, 4,060,568 and3,698,157 disclose specific methods for contacting a catalyst with amodifying compound containing silicon.

U.S. Pat. No. 4,548,914 describes another modification method involvingimpregnating catalysts with difficultly reducible oxides such as thoseof magnesium, calcium and/or phosphorus followed by treatment with watervapor to improve paraselectivity.

Steaming has been used in the preparation of zeolite catalysts to modifythe alpha or improve stability. For example, U.S. Pat. No. 4,559,314describes steaming a zeolite/binder composite at 200°-500° C. for atleast an hour to enhance activity by raising the alpha. U.S. Pat. No.4,522,929 describes presteaming a fresh zeolite catalyst so that thealpha activity first rises then falls to the level of the freshunsteamed catalyst, producing a stable catalyst which may be used inxylene isomerization. U.S. Pat. No. 4,443,554 describes steaminginactive zeolites (Na ZSM-5) to increase alpha activity. U.S. Pat. No.4,487,843 describes contacting a zeolite with steam prior to loadingwith a Group IIIB metal.

There has been no suggestion, however, to steam treat siliconpre-selectivated catalysts to enhance shape-selectivity. It has now beenfound that pre-selectivation treatment followed by steam treatment of amolecular sieve catalyst provides unexpectedly better results in shapeselective hydrocarbon conversions than pre-selectivation alone or steamtreatment alone. Furthermore, steaming alone has been found to bedetrimental in the context of the present invention.

Accordingly, it is an object of the invention to improve selectivity incatalytic molecular sieves thereby improving shape selectivity inhydrocarbon conversion processes over the molecular sieves.

SUMMARY OF THE INVENTION

The invention is a process for a shape selective hydrocarbon conversionby contacting a feed stream comprising the hydrocarbon, under conversionconditions, with a modified catalytic molecular sieve which has beenmodified by being pre-selectivated with a first silicon source andtreated with steam after it has been pre-selectivated.

The invention is also a process for disproportionation of toluene intoxylene which comprises contacting a reaction stream comprising toluene,at reaction conditions for converting toluene to xylene, with acatalytic molecular sieve having an initial Constraint Index of 1 to 12which has been modified by being pre-selectivated with a firstsilicon-containing compound, calcined and steamed. The stream may alsocontain a second silicon source which is a high efficiency para-xyleneselectivating agent to provide a single-pass para-xylene product purityrelative to all xylene product, of at least about 90% with at leastabout 15% toluene conversion.

The invention is also a method for modifying a catalytic molecular sievewhich includes pre-selectivating the molecular sieve with a firstsilicon-containing compound, calcining and steaming. Thepre-selectivated, calcined, steamed molecular sieve may be subsequentlycontacted with a mixture of a second silicon source which is ahigh-efficiency para-xylene selectivating agent and toluene at reactionconditions for converting toluene to xylene to provide a catalyst withgreatly enhanced paraselectivity.

The invention is also the catalyst modified by this method.

Advantageously, the modified, steamed catalyst has enhanced shapeselectivity for hydrocarbon conversions. Accordingly, thedisproportionation process of the invention has an increased toluenedisproportionation rate constant with increased selectivity forp-xylene.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is useful in shape selective hydrocarbonconversion reactions, for example, in converting various aromatics suchas toluene to commercially useful para-substituted benzenes, such aspara-xylene.

The catalytic molecular sieves useful herein have a Constraint Indexfrom about 1 to about 12 and include intermediate pore zeolites.Zeolites which conform to the specified values of Constraint Index forintermediate pore zeolites include ZSM-5, ZSM-11, ZSM-5/ZSM-11intermediate, ZSM-12, ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50,ZSM-57 and Zeolite Beta which are described, for example, in U.S. Pat.Nos. 3,702,886 and Re. No. 29,949, 3,709,979, 3,832,449, 4,046,859,4,556,447, 4,076,842, 4,016,245, 4,229,424, 4,397,827, 4,046,859,3,308,069 and Re. 28,341 and EP 127,399 to which reference is made fordetails of these zeolites. MCM-22, described in U.S. Pat. No. 4,973,784,is also useful herein.

A zeolite incorporated with a binder or in unbound form, is treated witha first silicon source. This first silicon treatment will be calledpre-selectivation. In pre-selectivation, the silicon compound isdeposited on the external surface of the catalyst by any suitablemethod. For example, the silicon may be dissolved in a solvent, mixedwith the catalyst, and then dried. The silicon compound employed may bein the form of a solution, a liquid or a gas under the conditions ofcontact with a zeolite. Examples of methods of depositing silicon on thesurface of the zeolite are found in U.S. Pat. Nos. 4,090,981, 4,127,616,4,465,886 and 4,477,583 to Rodewald, which are incorporated by referenceherein. The deposited silicon compound extensively covers the surfaceand resides substantially exclusively on the external surface of themolecular sieve.

The first silicon-containing compounds which are silicon sources for thepre-selectivation include alkoxy silanes, silanes and organoamine silanepolymers and also included are silicones which are the high efficiency,p-xylene selectivating agents discussed below which are also used assecond silicon sources for trim selectivation. Preferred siliconcontaining compounds for pre-selectivation include Si(OR)₄ whereinR=CH₃, C₂ H₅ or C₃ H₇ ; or a silicone polymer (SiO(R')₂)_(n) whereinR'=alkyl of C₁₋₁₀, aryl of C₆₋₁₀, or hydroxide and n is greater than 10and less than 1000; or an organoamine silane polymer where theorganoamine is --N(CH₃)₃, --N(C₂ H₅)₃ or --N(C₃ H₇)₃. The molecularsieve may be contacted with silicon-containing compound at a molecularsieve/silicon compound weight ratio of about 100/1 to about 1/100.

Following deposition of the first silicon-containing compound, thecatalyst is calcined. For example, the catalyst may be calcined in anoxygen-containing atmosphere, preferably air, at a rate of 0.2° to 5°C./minute to a temperature greater 300° C. but below a temperature atwhich the crystallinity of the zeolite is adversely affected. Generally,such temperature will be below 600° C. Preferably the temperature ofcalcination is within the approximate range of 350° to 550° C. Theproduct is maintained at the calcination temperature usually for 1 to 24hours.

After pre-selectivation and calcining, the catalyst is subjected tosteam treatment at a temperature of from about 200° C. to about 538° C.,preferably about 280° C. to about 400° C. with from about 5% to about100% steam at a pressure of from about 0.1 to about 50 psig, preferablyfrom about 50% to about 100% steam, for about two to about twelve hours,preferably from about three to about six hours.

The pre-selectivated molecular sieve catalyst, with or without binder,shows improved selectivity after steaming. If the catalyst is notpre-selectivated before steaming or if the catalyst is pre-selectivatedwithout steaming, as shown in the examples below, the same improvementin selectivity does not occur. Indeed, steaming alone can bedetrimental.

Shape Selective Conversions

Zeolites modified in accordance with the invention are generally usefulas catalysts in shape selective hydrocarbon conversion processesincluding cracking reactions involving dewaxing of hydrocarbonfeedstocks; isomerization of alkylaromatics; oligomerization of olefinsto form gasoline, distillate, lube oils or chemicals; alkylation ofaromatics; transalkylation of alkylaromatics; conversion of aromatics todialkyl-substituted benzene; conversion of oxygenates to hydrocarbons;rearrangement of oxygenates; and conversion of light paraffins andolefins to aromatics.

Dewaxing

The subject catalysts have good cracking and hydrocracking activity andmay be used to convert paraffins from high to low molecular weightsubstances in dewaxing processes. The catalysts of the invention may beused in processes such as those described, for example, in U.S. Pat.Nos. 3,700,585, Re. 28,398, 3,968,024 and 4,181,598 which areincorporated herein by references. The term dewaxing means the removalof those hydrocarbons which will readily solidify (waxes) from petroleumstocks. Hydrocarbon feeds which can be treated include lubricating oilstocks as well as those which have a freeze point or pour point problem,i.e., petroleum stocks boiling above 350° F. The dewaxing can be carriedout at either cracking or hydrocracking conditions.

In U.S. Pat. No. 3,700,585 and U.S. Pat. No. Re. 28,398 to Chen et al.,typical cracking conditions include a liquid hourly space velocity(LHSV) between about 0.5 and 200, a temperature between about 280° C.(550° F.) and 590° C. (1100° F.), a pressure between aboutsubatmospheric and several hundred atmospheric over ZSM-5 typecatalysts. Typical hydrocracking conditions include a liquid hourlyspace velocity between about 0.1 and 10, a temperature between about340° C. (650° F.) and 538° (1000° F.), a pressure between about 100 and3000 psig, and a hydrogen to hydrocarbon mole ratio between about oneand 20. U.S. Pat. No. 3,968,024 describes similar conversion using ZSM-5of small crystal size. U.S. Pat. No. 4,181,598 describes shape selectivecracking to produce lubes.

Isomerization of alkylaromatics

The modified catalysts of the invention are also advantageously used inthe isomerization of alkylaromatics in conversion reactions of the typedescribed, for example, in U.S. Pat. Nos. 3,856,872, 3,856,873, Re.30,157, 4,101,595, 4,101,597, 4,312,790, Re. 31,919 and 4,224,141 whichare herein incorporated by reference.

In U.S. Pat. No. 3,856,872 to Morrison, there is described a process forconverting C₈ aromatics xylene and ethylbenzene to para-xylene(octafining) at a temperature of 550° F. (288° C.) to 900° F. (482° C.),a pressure of 150 to 300 psig, and a liquid hour space velocity (LHSV)of 1 to 200 over an acid form catalyst containing metal such as platinumor nickel and hydrogen.

In U.S. Pat. No. 3,856,873 to Burress, mixtures of C₈ aromatichydrocarbons are isomerized to para-xylene by contact in vapor phasewith zeolite at a temperature of 500° F. (260° C.) to 1000° F. (538°C.), a pressure of 0 (atmospheric) to 1,000 psig, and a WHSV of 0.5 to250 with no added hydrogen. The catalyst is an acid ZSM-5, ZSM-12 orZSM-21.

U.S. Pat. No. 4,101,595 to Chen et al. describes the production ofpara-xylene from aromatics of 8 to 10 carbons over a dual functioncatalyst with a shape selective acid catalyzed step at a temperature of650° F. (343° C.) to 1000° F. (538° C.), a pressure of 50 to 500 psig, aLHSV of 0.1 to 100 and a molar of hydrogen/hydrocarbon of 0.1 to 15. Theacid form catalyst has a constraint index of 1 to 12, a silica/aluminaratio of at least 12, a crystal density of not less than 1.6 g/cc, maybe pre-coked, and includes Group VIII noble metal.

In U.S. Pat. No. 4,101,597 to Breckenridge, a C₈ feed is firstisomerized at 550° F. (288° C.) to 700° F. (371° C.) over a zeolitehaving a constraint index of 1 to 12, a silica/alumina ratio of at least12 and containing a metal having a hydrogenation/dehydrogenationfunction. A C⁹⁺ fraction produced during isomerization of C₈ isseparated from the other isomerization products, blended with hydrogenand toluene and contacted with a porous, acidic catalyst such as ZSM-5at 750° F. (399° C.) to 900° F. (482° C.). The catalyst has a constraintindex of 1 to 12, a silica/alumina ratio of at least 12, and a metalproviding hydrogenation/dehydrogenation function.

In U.S. Pat. No. 4,224,141 to Morrison, C₈ aromatics are isomerized tobenzene, toluene and xylenes over a ZSM-5 which is reduced in activityby dilution with inert matrix, steaming or thermal treatment, very highsilica/alumina ratio, base exchange with alkali metal, coking or thelike. The conversion is at a temperature of 800° F. (427° C.) to 1000°F. (538° C.) in a low pressure isomerization unit at a pressure onlysufficient to overcome pressure drop through downstream processingequipment, e.g. below 100 psig, and a WHSV of 1 to 200.

In U.S. Pat. No. 4,312,790 and Re. 31,919 to Butter et al., a zeolite isincorporated with noble metal subsequent to zeolite crystallization butprior to catalyst extrusion. The catalyst is used for xyleneisomerization at a temperature of 500° F. (260° C.) to 1000° F. (540°C.), a pressure between 50 and 1000 psig, a WHSV of 1 to 50 and ahydrogen/hydrocarbon mole ratio of 1 to 20.

Conversion of Oxygenates to Hydrocarbons

U.S. Pat. No. 4,476,330 to Kerr et al. describes the conversion ofaliphatic oxygenates to a higher molecular weight compound by contactingwith a zeolite having a silica/alumina ratio substantially greater than10 at a temperature of 70° F. (21° C.) to 1400° F. (760° C.). The feedsinclude lower aliphatic organic oxygenates up to C₆, acetals, ketals,acid halides, alcohols, carboxylic acids, aldehydes, acid anhydrides,epoxides, ethers, esters, hemiacetals, gem diols, hydroxy acids,ketones, ketenes, lactones, peracids, peroxides, sugars, and especiallyalcohols, ethers and esters.

Oligomerization of olefins

The modified catalysts of the invention are advantageously used in theoligomerization of olefins to form gasoline, distillate, lube oils orchemicals in conversion reactions of the type described, for example, inU.S. Pat. Nos. 4,517,399, 4,520,221, 4,547,609 and 4,547,613 which areherein incorporated by reference.

U.S. Pat. No. 4,517,399 to Chester et al. describes the conversion ofolefins of 3 to 18 carbons, e.g. propylene, to high viscosity, low pourpoint lubricating oils by contacting with ZSM-5 type zeolites havinglarge crystals of at least two microns. The conversion conditionsinclude a temperature of 350° F. (177° C.) to 650° F. (343° C.) apressure of 100 to 5000 psig, and a WHSV of 0.1 to 10.

U.S. Pat. No. 4,520,221 to Chen describes the polymerization of olefinsof 2 to 8 carbons, e.g. propylene, butylene, to high viscosity lubes,e.g. linear hydrocarbons, over highly siliceous, acidic ZSM-5 typecatalysts with surface acidity inactivated by treatment with base, e.g.bulky amines with a cross-section larger than about 5 Angstroms. Theconversion involves a one or two stage process with the polymerizationof lower olefins to linear materials, e.g. at about 200° C. over asurface poisoned zeolite, and oligomerization of the product over amodified or unmodified catalyst at a temperature of 50°-75° lower thanthe first stage, e.g. 150° C. Therefore, the temperatures range from 25°C. to 400° C., with a pressure of atmospheric to 1500 psi and a WHSV of0.04 to 1.0.

U.S. Pat. No. 4,547,609 to Dessau describes a two stage process wherebyin the first stage, light olefins of 2 to 6 carbons are oligomerized togasoline and distillate liquids including aliphatics of 10 to 20 carbonsover a zeolite having a crystal size greater than 0.5 micron atconditions including a temperature of 500° F. (260° C.) or higher, e.g.a range of 500° F. (260° C.) to 800° F. (437° C.), a pressure ofatmospheric to 2000 psig and a WHSV of 0.1 to 20. In the second stage,the distillate fraction is converted to high viscosity lubes by contactwith a zeolite of smaller crystal size under milder conditions of atemperature about 200° F. (100° C.) to 500° F. (260° C.), a pressure ofatmospheric to 650 psig, and a WHSV less than one.

U.S. Pat. No. 4,547,613 to Garwood et al. describes converting olefinsof 2 to 16 carbons to high viscosity lube oil. A ZSM-5 type catalyst ispre-conditioned by contact with light olefins of 2 to 16 carbons, e.g.propylene at 400° F. (204° C.) to 1000° F. (538° C.), at a pressure of 0to 100 psig for 1 to 70 hours. Conversion conditions include atemperature of 350° F. (177° C.) to 650° F. (343° C.), a pressure of 400to 5000 psig and a WHSV of 0.1 to 10. The lube fraction may be subjectedto a hydrogenation step to stabilize.

Conversion of aromatics to dialkyl-substituted benzene

The modified zeolite catalysts of the invention are advantageously usedin the conversion of aromatics compounds to provide dialkyl-substitutedbenzene products which are highly enriched in the para-dialkylsubstituted benzene isomer. Conversion reactions of this type includearomatics alkylation, transalkylation and disproportionation. Aromaticsalkylations in which the catalysts of the invention can be used aredescribed, for example, in U.S. Pat. Nos. 3,755,483, 4,086,287,4,117,024 and 4,117,026 which are herein incorporated by reference.

As described in U.S. Pat. No. 3,755,483 to Burress, aromatichydrocarbons such as benzenes, naphthalenes, anthracenes and substitutedderivatives thereof, e.g. toluene and xylene, may be alkylated withalkylating agents such as olefins ethylene, propylene, dodecylene, andformaldehyde, alkyl halides, and alkyl alcohols with 1 to 24 carbonsunder vapor phase conditions including a reactor inlet temperature up toabout 900° F. (482° C.), with a reactor bed temperature up to about1050° F. (566°), at a pressure of about atmospheric to about 3000 psig,a ratio of aromatic/alkylating agent of about 1:1 to about 20:1 and aWHSV of 20 to 3000 over ZSM-12.

As described in U.S. Pat. No. 4,086,287 to Kaeding et al.,monoalkylbenzenes having alkyls of 1-2 carbons, such as toluene andethylbenzene, may be ethylated to produce a para-ethyl derivative, e.g.para-ethyltoluene at a temperature of from about 250° C. to about 600°C., a pressure of 0.1 atmospheres to 100 atmospheres, a weight hourlyspace velocity (WHSV) of 0.1 to 100, and a ratio of feed/ethylatingagent of 1 to 10 over a catalyst having an acid activity, i.e., alpha,of 2 to 5000, modified by precoking or combining with oxides ofphosphorus, boron or antimony to attain a catalyst with a xylenesorption capacity greater than 1 g/100 g of zeolite and an ortho xylenesorption time for 30% of said capacity of greater than 10 minutes, wheresorption capacity and sorption time are measured at 120° C. and a xylenepressure of 4.5±0.8 mm of mercury.

U.S. Pat. No. 4,117,024 to Kaeding describes a process for theethylation of toluene or ethylbenzene to produce p-ethyltoluene at atemperature of 350° C. to 550° C., a critical pressure of greater thanone atmosphere and less than 400 psig, with hydrogen/ethylene ratio of0.5 to 10 to reduce aging of the catalyst. The zeolite described in U.S.Pat. No. 4,117,024 has a crystal size greater then one micron, and ismodified as the catalyst in U.S. Pat. No. 4,086,287 to attain thesorption capacity described in U.S. Pat. No. 4,086,287.

U.S. Pat. No. 4,117,026 to Haag and Olson describes the production ofpara-dialkyl benzenes having alkyls of 1 to 4 carbons under conditionswhich vary according to the feed. When the feed includes monoalkylsubstituted benzenes having an alkyl of 1 to 4 carbons, olefins of 2 to15, or paraffins of 3 to 60 carbons or mixtures thereof, conversionconditions include a temperature of 250° C. to 750°, a pressure of 0.1to 100 atmospheres and a WHSV of 0.1 to 2000. For the disproportionationof toluene, the conditions include a temperature of 400° C. to 700° C.,a pressure of 1 to 100 atmospheres and a WHSV of 1-50. When the feedincludes olefins of 2 to 15 carbons including cyclic olefins, theconversion conditions include a temperature of 300° C. to 700° C., apressure of 1 to 100 atmospheres and a WHSV of 1 to 1000. When the feedincludes paraffins of 3 to 60 carbons, conditions include a temperatureof 300° C. to 700° C., a pressure of 1 to 100 atmospheres and a WHSV of0.1 to 100. However for lower paraffins of 3 to 5 carbons, thetemperature should be above 400° C. When the feed includes mixedaromatics such as ethylbenzene and toluene, and also optionally olefinsof 2 to 20 carbons or paraffins of 5 to 25 carbons, conversionconditions includes a temperature of 250° C. to 500° C. and a pressuregreater than 200 psig. In the absence of added aromatics, the olefinsand higher paraffins are more reactive and require lower severity ofoperation, e.g. a temperature of 250° C. to 600° C., preferably300°-550° C. The catalyst described in U.S. Pat. No. 4,117,026 ismodified as in U.S. Pat. No. 4,086,287. Conversion of light paraffinsand olefins to aromatics

The modified catalysts of the invention may also be used in theconversion of light paraffins and olefins to aromatics in processes ofthe type described, for example, in U.S. Pat. Nos. 3,760,024 and3,756,942 which are herein incorporated by reference.

U.S. Pat. No. 3,760,024 to Cattanach describes a process for theconversion of paraffins of 2 to 4 carbons and/or olefins to aromatics of6 to 10 carbons over a ZSM-5 type catalyst with or withouthydrogenation/dehydrogenation component. Conversion conditions include atemperature of 100° C. to 650° C., a pressure of 0 to 1,000 psig, a WHSVof 0.1 to 500 and a hydrogen/hydrocarbon ratio of 0 to 20.

U.S. Pat. No. 3,756,942 to Cattanach describes the conversion ofparaffins, olefins and naphthenes to aromatics over ZSM-5 typecatalysts. If the feed contains at least 35 wt. % olefins, conversion isat 650° F. (363° C.) to 1400° F. (760° C.). If the feed contains lessthan 35 wt. % olefins, the temperature is 900° F. (482° C.) to 1400° F.(760° C.) with the absence of substantial added hydrogen. For both typesof feed, the pressure is atmospheric to 35 atmospheres and the WHSV 1 to15.

Therefore, the modified catalysts of the present invention are suitablefor use in a variety of shape selective hydrocarbon conversion processesincluding as non-limiting examples, cracking hydrocarbons with reactionconditions including a temperature of from about 300° C. to about 700°C., a pressure of from about 0.1 atmosphere (bar) to about 30atmospheres and a weight hourly space velocity of from about 0.1 hr⁻¹ toabout 20 hr⁻¹ ; dehydrogenating hydrocarbon compounds with reactionconditions including a temperature of from about 300° C. to about 700°C., a pressure of from about 0.1 atmosphere to about 10 atmospheres, anda weight hourly space velocity of from about 0.1 to about 20; convertingparaffins to aromatics with reaction conditions including a temperatureof from about 300° C. to about 700° C., a pressure of from about 0.1atmosphere to about 60 atmospheres, a weight hourly space velocity offrom about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio offrom about 0 to about 20; converting olefins to aromatics, e.g. benzene,toluene and xylene, with reaction conditions including a temperature offrom about 100° C. to about 700° C., a pressure of from about 0.1atmosphere to about 60 atmospheres, a weight hourly space velocity offrom about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio offrom about 0 to about 20; converting alcohols, e.g. methanol, or ethers,e.g. dimethylether, or mixtures thereof to hydrocarbons includingolefins and/or aromatics with reaction conditions including atemperature of from about 275° C. to about 600° C., a pressure of fromabout 0.5 atmosphere to about 50 atmospheres and a liquid hourly spacevelocity of from about 0.5 to about 100; isomerizing xylene feedstockcomponents with reaction conditions including a temperature of fromabout 230° C. to about 510° C., a pressure of from about 3 atmospheresto about 35 atmospheres, a weight hourly space velocity of from about0.1 to about 200 and a hydrogen/hydrocarbon mole ratio of from about 0to about 100; disproportionating toluene with reaction conditionsincluding a temperature of from about 200° C. to about 760° C., apressure from about atmospheric to about 60 atmospheres and a weighthourly space velocity of from about 0.08 to about 20; alkylatingaromatic hydrocarbons, e.g. benzene and alkylbenzenes in the presence ofan alkylating agent, e.g. olefins, formaldehyde, alkyl halides andalcohols, with reaction conditions including a temperature of from about250° C. to about 500° C., a pressure of from about atmospheric to about200 atmospheres, a weight hourly space velocity of from about 2 to about2000 and an aromatic hydrocarbon/alkylating agent mole ratio of fromabout 1/1 to about 20/1; and transalkylkating aromatic hydrocarbons inthe presence of polyalkylaromatic hydrocarbons with reaction conditionsincluding a temperature of from about 340° C. to about 500° C., apressure of from about atmospheric to about 020 atmospheres, a weighthourly space velocity of from about 10 to about 1000 and an aromatichydrocarbon/polyalkylaromatic hydrocarbon mole ratio of from about 1/1to about 16/1.

In general, therefore, catalytic conversion conditions over a catalystcomprising the modified zeolite include a temperature of from about 100°C. to about 760° C., a pressure of from about 0.1 atmosphere (bar) toabout 200 atmospheres (bar), a weight hourly space velocity of fromabout 0.08 hr⁻¹ to about 2000 hr⁻¹ and a hydrogen/organic, e.g.hydrocarbon compound of from 0 to about 100.

Toluene Disproportionation

Toluene Disproportionation will be used as a representative shapeselective conversion. A catalyst treated in the manner described hereinhas a desirable decreased ortho-xylene sorption rate parameter andyields a para-selective product in toluene disproportionation.

Diffusion rate constants in toluene disproportionation have beendiscussed by D. H. Olson and W. O. Haag, "Structure-SelectivityRelationship in Xylene Isomerization and Selective TolueneDisproportionation" Catalytic Materials Relationship Between Structureand Reactivity, ACS Symposium Ser. No. 248 (1984).

In toluene disproportionation, toluene diffuses into the zeolite with adiffusivity D_(T). The toluene undergoes disproportionation to p-, m-,and o-xylene and benzene at a total rate constant k_(D) : ##EQU1##

The degree of para-selectivity depends on the activity and the diffusioncharacteristics of the catalyst. The primary product will be rich in thepara isomer if initially produced m- and o- xylene diffuse out of thezeolite crystal at a lower rate (D_(m),o /r²) than that of theirconversion to p-xylene (k_(I)) and the p-xylene diffusion (D_(p) /r²)out of the catalyst

D_(m) =diffusion of m-xylene

D_(o) =diffusion of o-xylene

D_(p) =diffusion of p-xylene

r=length of diffusion path (crystal radius)

k_(I) =rate of interconversion via isomerization of xylene isomersyielding secondary xylene product m-xylene and o-xylene.

It is desirable to reduce the k_(I) thereby reducing the isomerizationof p-xylene to o- and m-xylene in a secondary reaction by adjustingD_(m),o /r² downward so that ##EQU2## Thus a para-rich primary productwill result. It is therefore apparent that if the o-xylene diffusion(D_(o) /r²) can be adjusted downward, the p-xylene product willincrease.

Selective Toluene Disproportionation may be carried out under conditionswhich include a temperature between about 200° C. and about 600° C., apressure from about 0.1 psig to about 1000 psig, a weight hourly spacevelocity (WHSV) from about 0.1 hr⁻¹ to about 20 hr⁻¹ and ahydrogen/hydrocarbon mole ratio from 0 to about 20.

The catalyst may be pre-selectivated ex situ with a firstsilicon-containing compound as described above, calcined then steamedand optionally trim selectivated with a second silicon-containingcompound which is termed a high efficiency para-xylene selectivatingagent.

The invention involves the disproportionation of toluene. However, thepresent invention also applies to other methylation reactions such asthose using methylhalides and methylethers. Normally a single passconversion of a toluene stream results in a product stream whichincludes dimethylbenzenes having alkyl groups at all locations, i.e.,ortho-, meta-, and para-xylenes. Furthermore, the xylenes are known toproceed in a reaction which produces unwanted ethylbenzenes (EB) by thefollowing reaction: ##STR2##

By deposition of platinum on the molecular sieve, the presence ofethylbenzene can be reduced.

As explained in greater detail herein, the present invention provides aprocess for obtaining p-xylene at toluene conversion rates of at least15%, preferably at least about 20-25%, with a para-xylene purity ofgreater than 85%, preferably at least 90%.

The present invention also comprises the regioselective conversion oftoluene to para-xylene by methylating toluene in a reaction streamcontaining a toluene feed with a trim selectivated catalytic molecularsieve which has been pre-selectivated and steamed. The trimselectivation method is described below. As used herein, the term"para-xylene purity" means the percentage of para-xylene in all of thexylene products para-xylene, ortho-xylene, and meta-xylene. Thoseskilled in the art will appreciate that the proximity of the boilingpoints of these xylene products necessitates more expensive separationprocesses whereas para-xylene may be more readily separated from othercomponents in the product stream such as benzene, toluene, andpara-ethyltoluene.

As used herein, the term "xylene-conversion product" indicates the totalamount of xylenes resulting from the disproportionation reaction. Theword "para-xylene" in this term is not intended to limit the scope ofthe present invention to the production of xylenes since otherpara-substituted aromatics may be produced.

The invention also comprises a method for the regioselective productionof para-xylene by passing a reaction stream which contains an aromaticfeedstock, e.g. toluene, in a single pass, over a pre-selectivated,steamed, then trim-selectivated catalytic molecular sieve, the singlepass in the presence of hydrogen at reaction conditions suitable toprovide para-xylene purity of greater than about 80%, preferably greaterthan 90%. The product stream may also include small amounts of ortho-and meta-xylene and trace amounts of impurities such as ethylbenzene.

The toluene may be fed simultaneously with a second silicon source whichis a high-efficiency selectivating agent and hydrogen at reactionconditions until the desired p-xylene selectivity is attained, whereuponthe feed of selectivating agent is discontinued. This co-feeding ofselectivating agent with toluene will be termed "trim selectivation".Reaction conditions for this trim-selectivation step generally include atemperature of about 350°-540° C. and a pressure of about atmospheric-5000 psig. The feed is provided to the system at a rate of about 0.1-20WHSV. The hydrogen is fed at a hydrogen to hydrocarbon molar ratio ofabout 0.1-20.

The high efficiency para-xylene selectivating agent for trimselectivation preferably comprises a silicon containing compounddiscussed in greater detail below. For example, organic silicons such asphenylmethyl silicone, dimethyl silicone, and mixtures thereof aresuitable. According to one preferred embodiment of the presentinvention, a silicone containing phenylmethylsilicone anddimethylsilicone groups in a ratio of about 1:1 is co-fed to the system,while the other components, e.g. toluene and hydrogen, are fed in theamounts set forth above. The high-efficiency para-xylene selectivatingagent is fed in an amount of about 0.1%-50% of the toluene according toone embodiment. Depending upon the percentage of selectivating agentused, the trim selectivation will last for at least one hour, or about50-300 hours, most preferably less than 170 hrs.

As used herein, the term "high efficiency, p-xylene selectivating agent"may be used for both trim selectivation and pre-selectivation and isused to indicate substances which will increase the para-selectivity ofa catalytic molecular sieve to the stated levels in toluenedisproportionation while maintaining commercially acceptable toluene toxylene conversion levels. Such substances include, for example, organicsilicon compounds such as phenylmethyl silicone, dimethylsilicone, andblends thereof which have been found to be suitable.

The trim selectivation of the catalyst is preferably performed with asecond silicon containing compound which is herein termed a highefficiency, p-xylene selectivating agent. These selectivating agentsinclude silicone compounds which can be characterized by the generalformula: ##STR3## where R₁ is hydrogen, fluorine, hydroxy, alkyl,aralkyl, alkaryl or fluoro-alkyl. The hydrocarbon substituents generallycontain from 1 to 10 carbon atoms and preferably are methyl or ethylgroups. R₂ is selected from the same group as R₁, and n is an integer ofat least 2 and generally in the range of 3 to 1000. The molecular weightof the silicone compound employed is generally between about 80 andabout 20,000 and preferably within the approximate range of 150 to10,000. Representative silicone compounds include dimethylsilicone,diethylsilicone, phenylmethylsilicone, methylhydrogensilicone,ethylhydrogensilicone, phenylhydrogensilicone, methylethylsilicone,phenylethylsilicone, diphenylsilicone, methyltrifluoropropylsilicone,ethyltrifluoropropysilicone, polydimethylsilicone,tetrachlorophenylmethyl silicone, tetrachlorophenylethyl silicone,tetrachlorophenylhydrogen silicone, tetrachlorophenylphenyl silicone,methylvinylsilicone and ethylvinylsilicone. The silicone compound neednot be linear but may be cyclic as for examplehexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane,hexaphenylcyclotrisiloxane and octaphenylcyclotetrasiloxane. Mixtures ofthese compounds may also be used as well as silicones with otherfunctional groups. Other silicon-containing compounds, such as silanes,may also be utilized.

Preferably, the kinetic diameter of the high efficiency, p-xyleneselectivating agent is larger than the zeolite pore diameter, in orderto avoid reducing the internal activity of the catalyst.

While not wishing to be bound by theory, it is believed that advantagesof the present invention are in part obtained by rendering acid sites onthe external surfaces of the catalyst substantially inaccessible toreactants while increasing catalyst tortuosity. Acid sites existing onthe external surface of the catalyst are believed to isomerize thepara-xylene exiting the catalyst pores back to an equilibrium level withthe other two isomers, thereby reducing the amount of para-xylene in thexylenes to only about 24%. By reducing the availability of these acidsites to the para-xylene exiting the pores of the catalyst, therelatively high level of para-xylene can be maintained. It is believedthat the high-efficiency, p-xylene selectivity agents of the presentinvention block or otherwise render these external acid sitesunavailable to the para-xylene by chemically modifying said sites.

In line with this theory, it is also believed that the presence ofhydrogen in the reaction zone during the trim selectivation is importantin order to maintain the desired high yields of para-xylene when asilicone compound is used as the high-efficiency para-xyleneselectivating agent. The importance of the hydrogen may be reduced inalternative embodiments by using a high efficiency para-xyleneselectivating agent comprising silane or some other compound whicheffectively renders the isomerizing acid sites on the external surfaceof the catalyst inaccessible.

One process of the present invention utilizes a high efficiencypara-xylene selectivating agent which includes a silicon compoundwherein the silicon compound is introduced by co-feeding, for example,at least one silicon compound with the toluene feedstock over aconversion catalyst at reaction conditions until the desired degree ofselectivation is achieved, at which time the feed of selectivating agentmay be discontinued.

The toluene feedstock preferably includes about 50% to 100% toluene,more preferably at least about 80% toluene in the toluene feedstock.Other compounds such as benzene, xylenes, and trimethylbenzene may alsobe present in the toluene feedstock without adversely affecting thepresent invention.

The toluene feedstock may also be dried, if desired, in a manner whichwill minimize moisture entering the reaction zone. Methods known in theart suitable for drying the toluene charge for the present process arenumerous. These methods include percolation through any suitabledesiccant, for example, silica gel, activated alumina, molecular sievesor other suitable substances, or the use of liquid charge dryers.

The catalytic molecular sieves for the present invention are preferablyin the hydrogen form but may be in the ammonium or sodium form andpreferably comprise an intermediate pore-size zeolite such as a ZSM-5,ZSM-11, ZSM-22, ZSM-23, or ZSM-35 as discussed above. The catalystpreferably has an alpha value greater than 100, for example about150-2000, and a silica-alumina ratio less than 1000 preferably about20-500. The Alpha Value of the catalyst may be increased by initiallytreating the catalyst with nitric acid or by mild steaming beforepre-selectivation. This type of steaming is discussed in U.S. Pat. No.4,326,994.

The Alpha Value is an approximate indication of the catalytic crackingactivity of the catalyst compared to a standard catalyst and it givesthe relative rate constant (rate of normal hexane conversion per volumeof catalyst per unit time.) It is based on the activity of the amorphoussilica-alumina cracking catalyst taken as an Alpha of 1 (RateConstant=0.016 sec⁻¹). The Alpha Test is described in U.S. Pat. No.3,354,078 and in The Journal of Catalysis, Vol. 4, pp. 522-529 (August1965): Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980), eachincorporated herein by reference as to that description. It is notedthat intrinsic rate constants for many acid-catalyzed reactions areproportional to the Alpha Value for a particular crystalline silicatecatalyst (see "The Active Site of Acidic Aluminosilicate Catalysts,"Nature, Vol. 309, No. 5959, pp. 589-591, 14 Jun. 1984). The experimentalconditions of the test used herein include a constant temperature of538° C. and a variable flow rate as described in detail in the Journalof Catalysis, Vol. 61, p. 395. The catalytic molecular sieves alsopreferably have a Constraint Index of about 1-12. The method by whichConstraint Index is determined is described fully in U.S. Pat. No.4,016,218, incorporated herein by reference for details of the method.The crystal size of zeolites used herein is preferably greater than 0.1micron.

For the improved disproportionation process of this invention, thesuitable molecular sieve may be employed in combination with a supportor binder material such as, for example, a porous inorganic oxidesupport or a clay binder. While the preferred binder is silica, othernon-limiting examples of such binder materials include alumina,zirconia, magnesia, thoria, titania, boria and combinations thereof,generally in the form of dried inorganic oxide gels or gelatinousprecipitates. Suitable clay materials include, by way of example,bentonite and kieselguhr. The relative proportion of suitablecrystalline molecular sieve to the total composition of catalyst andbinder or support may be about 30 to about 90 percent by weight and ispreferably about 50-80 percent by weight of the composition. Thecomposition may be in the form of an extrudate, beads or fluidizablemicrospheres.

Operating conditions employed in the process of the present inventionwill affect the para-selectivity and toluene conversion rate. Suchconditions include the temperature, pressure, space velocity, molarratio of the reactants, and the hydrogen to hydrocarbon mole ratio. Apre-selectivated and steamed catalytic molecular sieve may be contactedwith a toluene feedstock which includes a silicone compound underconditions for effecting vapor-phase disproportionation. Conditionseffective for accomplishing the high para-selectivity and acceptabletoluene disproportionation conversion rates include a reactor inlettemperature of about 350°-540° C., preferably greater than about 400°C., a pressure of about atmospheric -5000 psig, preferably about 100 to1000 psig, a WHSV of about 0.1-20, preferably about 2-10, and a hydrogento hydrocarbon mole ratio of about 0.1-20, preferably about 2-6. Thisprocess may be conducted in either batch or fluid bed operation withattendant benefits of either operation readily obtainable. The effluentmay be separated and distilled to remove the desired product, i.e.,para-xylene, plus other by-products.

The catalyst may be further modified in order to reduce the amount ofundesirable by-products, particularly ethylbenzene. The state of the artis such that the reactor effluent from standard toluenedisproportionation typically contains about 0.5% ethylbenzeneby-product. Upon distillation of the reaction products, the level ofethylbenzene in the C₈ fraction often increases to about 3-4 percent.This level of ethylbenzene is unacceptable for polymer grade p-xylenesince ethylbenzene in the C₈ product, if not removed, degrades thequality of fibers ultimately produced from the p-xylene product.Consequently, ethylbenzene content must be kept low. The specificationfor ethylbenzene in the C₈ product has been determined by industry to beless than 0.3%. Ethylbenzene can be substantially removed byisomerization or by superfractionation processes. Removal of theethylbenzene by conventional isomerization may be impractical when thexylene stream includes greater than 70 or 80 percent para-xylene, sincethe para-xylene would be concurrently isomerized to equilibrium xylenesreducing the amount of para-xylene in this xylene stream. It is known inthe art that the alternative procedure of removing the ethylbenzene bysuperfractionation is extremely expensive.

In order to avoid the need for downstream ethylbenzene removal, thelevel of ethylbenzene by-product is advantageously reduced byincorporating a hydrogenation-dehydrogenation function in the catalyst,such as by addition of a metal compound such as platinum. While platinumis the preferred metal, other metals such as palladium, nickel, copper,cobalt, molybdenum, rhodium, ruthenium, silver, gold, mercury, osmium,iron, zinc, cadmium, and mixtures thereof may be utilized. The metal maybe added by cation exchange, in amounts of about 0.01-2%, typicallyabout 0.5%. The metal must be able to enter the pores of the catalyst inorder to survive a subsequent calcination step. For example, a platinummodified catalyst can be prepared by first adding the catalyst to asolution of ammonium nitrate in order to convert the catalyst to theammonium form. The catalyst is subsequently contacted with an aqueoussolution of tetraamine platinum(II) nitrate or tetraamine platinum(II)chloride. The metallic compound advantageously enters the pores of thecatalyst. The catalyst can then be filtered, washed with water andcalcined at temperatures of about 250° to 500° C.

The following non-limiting examples illustrate the invention.

In the examples, the o-xylene sorption rate parameter D_(o) /r² wasmeasured at 120° C. and 3.9 torr.

D_(o) =diffusivity of o-xylene

r=crystal size

D_(o) /r² =the diffusion rate constant (K) which is the rate thato-xylene diffuses out of the crystal

COMPARATIVE EXAMPLES 1-3 EXAMPLE 1

HZSM-5/Al₂ O₃ was steamed at 342° C. for 3 hours with 100% steam thentested for toluene disproportionation. One atmosphere toluene wasreacted with the steamed ZSM-5/Al₂ O₃ at 482° C. and the tolueneconversion changed by varying the toluene WHSV. The p-xylene selectivityof the unsteamed catalyst was 37.5% at 4% toluene conversion with a TDP(toluene disproportionation) rate constant (K) of 163. After steamingthe p-xylene selectivity decreased to 30.6% at 4% toluene conversion anda TDP rate constant of 341.

EXAMPLES 2-3

Additional samples of HZSM-5/Al₂ O₃ were steamed and tested for toluenedisproportionation and TDP rate constant. The results are shown in Table1.

                  TABLE 1                                                         ______________________________________                                                  Unsteamed        Steamed                                                      K    p-sel*      K      p-sel                                       ______________________________________                                        Example 2   190    35.0        363  30.0                                      Example 3   176    36.9        384  30.0                                      ______________________________________                                         *p-sel = para selectivity                                                

The results of Examples 1-3 show that steaming a non-pre-selectivatedcatalyst decreases para-selectivity.

EXAMPLE 4

One gram of water soluble n-propylamine silane polymer was diluted with1 gram deionized (DI) H₂ O. One gram of as-synthesized ZSM-5 was mixedwith 1 gram of the silane polymer/H₂ O solution at room temperature for2 hours. The sample was dried at 130° C. then calcined in nitrogenfollowed by air at 538° C.

The o-xylene sorption was measured at 120° C. before and aftertreatment. The D/r² had decreased from 8.5×10⁻⁶ s⁻¹ to 6.5×10⁻⁷ s⁻¹.

EXAMPLE 5

A silica modified HZSM-5 catalyst prepared as described in Example 4 wastested for toluene disproportionation. One atmosphere toluene wasreacted with the catalyst at 482° C. The conversion was changed byvarying the WHSV. 42.9% p-xylene selectivity was obtained at 4% tolueneconversion with a rate constant of 105.

EXAMPLE 6

The silica modified HZSM-5 prepared as described in Example 4 wassteamed at 342° C. and 100% H₂ O(g) for three hours. After steaming thesample was tested for toluene disproportionation as in Example 5. 73.2%p-selectivity was obtained at 4% toluene conversion. The steam treatmentincreased the p-xylene selectivity from 42.9% to 73.2%. The rateconstant for toluene disproportionation increased after steaming from105 to 150. The o-xylene sorption rate parameter, D/r², had decreasedfrom 6.5×10⁻⁷ s⁻¹ to 1.85×10⁻⁷ s⁻¹.

EXAMPLE 7

One gram of water soluble n-propylamine silane polymer was diluted with5 grams deionized water. One gram of HZSM-5/Al₂ O₃ was mixed with 1 gramof the silane polymer/H₂ O solution at room temperature for 2 hours. Thesample was dried at 130° C. then calcined in nitrogen followed by air at538° C.

The o-xylene sorption parameter was measured before and after silanetreatment. The D/r² decreased from 4.8×10⁻⁶ s⁻¹ before treatment to6.7×10⁻⁷ s⁻¹ after treatment.

EXAMPLE 8

The silica modified HZSM-5/Al₂ O₃ prepared in Example 7 was steamed at342° C. and 100% H₂ O(g) for three hours. The sample was tested fortoluene disproportionation before and after steaming. Steam treatmentincreased the p-xylene selectivity from 40.5% to 67.9% at 4% tolueneconversion. The rate constant for toluene disproportionation (k_(D))increased after steaming from 91 to 195.

EXAMPLE 9

Ten grams of water soluble n-propylamine silane polymer was diluted with10 grams deionized (DI) H₂ O . Five grams of ZSM-5/SiO₂ was mixed withfive grams of the silane solution at room temperature for 2 hours. Thesample was dried at 130° C. then calcined in nitrogen followed by air at538° C.

The o-xylene sorption rate was measured before and after silanetreatment. The D_(o) /r² decreased from 1.5×10⁻⁴ s⁻¹ before treatment to9.0×10⁻⁷ s⁻¹ after treatment.

EXAMPLE 10

The modified ZSM-5/SiO₂ prepared in Example 9 was steamed at 342° C. and100% H₂ O (g) for three hours.

The o-xylene sorption rate was measured. The D_(o) /r² decreased from9.0×10⁻⁷ s⁻¹ before treatment to 5.6×10⁻⁷ s⁻¹ after treatment.

EXAMPLE 11

A silica modified ZSM-5/SiO₂ catalyst prepared as described in U.S. Pat.No. 4,090,981 was steamed at 342° C. for 3 hours with 100% steam. Thecatalyst was tested before and after steaming. At 446° C., 8 WHSV, 2 H₂/HC and 500 psi, the steamed silica modified ZSM-5/SiO₂ showed 93.4%p-xylene selectivity at 18.9% toluene conversion while the unsteamedcatalyst showed 79.2% p-xylene selectivity at 18.3% toluene conversion.

EXAMPLE 12

A silica modified ZSM-5/Al₂ O₃ catalyst prepared as described in U.S.Pat. No. 4,090,981 was steamed at 342° C. for 3 hours with 100% steam.After steaming, the sample was tested for toluene disproportionation asdescribed in Example 5. After steaming, the p-xylene selectivityincreased from 60.6% to 86.7% at 4% toluene conversion. The rateconstant for toluene disproportionation increased from 149 to 238.

EXAMPLE 13

NaZSM-5/SiO₂ was treated with a propylamine silane polymer/H₂ O mixture,7:1 wt ratio, at room temperature for overnight. The sample was filteredand dried at 130° C. and then calcined at 538° C. in nitrogen followedby air.

The calcined sample was exchanged 2-3 times with 1M NH₃ NO₃ at roomtemperature for 1-2 hours. The o-xylene diffusivity was 3×10⁻⁷ s⁻¹. NH₃-TPAD was performed to determine the number of acid sites which was 0.43meq/g.

The exchanged SiO₂ -ZSM-5/SiO₂ material was calcined in air at 538° C.and then steamed at 315° C. for 3 hours in 100% steam.

Catalytic evaluation of selectivated-steamed catalyst was conducted inan automated unit with on line sampling. One gram of 14/30 mesh steamedmaterial was loaded into a 0.305" stainless steel tube reactor. Thesample was heated to 446° C. in 40 cc/min H₂ at a heating rate of 3.5°C./min. Pure toluene was then introduced at 446° C., 4, 8, 16 and 32WHSV, 2 H₂ /HC and 500 psi to measure the catalytic performance. Asolution of 1 wt. % phenylmethyl-dimethyl silicone copolymer (Dow 550)in toluene was then passed over the catalyst at 466° C., 4 WHSV, 2 H₂/HC and 500 psi for 4 hours. To determine the activity/selectivityperformance of the selectivated catalysts, reactor temperature wasvaried to change toluene conversion. Toluene conversion/p-xyleneselectivities are shown below in Table 2.

                  TABLE 2                                                         ______________________________________                                        Temp, °C.    446    466                                                WHSV                4      4                                                  Pres, psi           500    500                                                H.sub.2 /HC         2      2                                                  Toluene Conv., %    15.5   27.1                                               p-xylene sel., %    96.9   91.4                                               ______________________________________                                    

EXAMPLE 14

a. Five grams of propylamine silane polymer were diluted with 5 grams DIH₂ O. 10 grams of Na-ZSM-5/SiO₂ were treated with 10 grams of thepropylamine silane polymer/H₂ O solution by impregnation for overnightand then dried at 130° C.

The sample was then calcined in 300 cc/min N₂ using a heating rate of 2°C./min to 538° C. then held at 538° C. for 2 hours followed by 300cc/min air heated at 2° C./min from 300° C. to 538° C. then held for 2hours.

The o-xylene diffusivity was measured at 120° C. The selectivationprocedure reduced the D/r² from 1.5×10⁻⁴ s⁻¹ to 1×10⁻⁵ s⁻¹. To reducethe diffusivity further a second selectivation was performed.

b. 7 grams of propylamine silane polymer were diluted with 3 grams DI H₂O. 10 grams of SiO₂ //Na-ZSM-5/SiO₂ described in a. above were treatedwith 10 grams of the propylamine silane polymer/H₂ O solution byimpregnation for overnight and then dried at 130° C. The sample was N₂/air calcined as described above.

The calcined sample was exchanged 2-3 times with 1M NH₄ NO₃ at roomtemperature for 1-2 hours to reduce the sodium content to less than 500ppm. The sample was calcined in air at 538° C. for 2 hours then steamedat 343° C. in 100% steam. Representative catalyst selectivities areshown below after 6 hours trim selectivation with 1%methylphenyl-dimethylsilicone in toluene was then passed over thecatalyst at 466° C., 4 WHSV, 2 H₂ /HC and 500 psi for 6 hours. Tolueneconversion/p-xylene selectivities are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Temp, °C.    466    446                                                WHSV                4      8                                                  Pres, psi           500    500                                                H.sub.2 /HC         2      2                                                  Toluene Conv., %    30.9   17.1                                               p-xylene sel., %    90.2   93.5                                               ______________________________________                                    

What is claimed is:
 1. A method for modifying a catalytic molecularsieve comprising pre-selectivating the molecular sieve with a firstsilicon-containing compound selected from the group consisting ofsilicones, tetraalkoxy silanes, and organoamine silane polymers,calcining and subsequently steaming at about 200° C. to about 400° C. 2.A method of claim 1 wherein the molecular sieve comprises a zeolitehaving an initial Constraint Index of from about 1 to about
 12. 3. Themethod of claim 1 wherein the molecular sieve is incorporated withbinder before being modified.
 4. The method of claim 1 wherein themolecular sieve is incorporated with binder after being modified.
 5. Themethod of claim 1 wherein the silicon-containing compound is selectedfrom the group consisting ofSi(OR)₄, wherein R is selected from thegroup consisting of CH₃, C₂ H₅ and C₃ H₇ ; (SiO(R')₂)_(n), wherein R' isselected from the group consisting of alkyl of C₁₋₁₀, aryl of C₆₋₁₀,hydroxide and hydrogen and n is greater than 10 and less than 1000; andorganoamine silane polymer, wherein the organoamine is selected from thegroup consisting of --N(CH₃)₃, --N(C₂ H₅)₃ and --N(C₃ H₇)₃.
 6. Themethod of claim 1 wherein the steaming comprises treating the molecularsieve with steam under conditions comprising between about five andabout 100 percent water vapor, and a pressure of from about 0.1 to about50 psig, for a time of about two to about 12 hours.
 7. The method ofclaim 1 wherein the molecular sieve is modified in an as-synthesizedcondition.
 8. The method of claim 1 further comprising contacting thepre-selectivated calcined, steamed catalytic molecular sieve with amixture comprising toluene and a second silicon source which is a,paraxylene selectivating agent at reaction conditions for conveningtoluene to xylene for at least one hour to yield a twice modifiedmolecular sieve.
 9. The method of claim 8 wherein the second siliconsource is a silicone.
 10. The method of claim 9 wherein the siliconecomprises a mixture of phenylmethylsilicone and dimethylsilicone.
 11. Acatalyst comprising:a molecular sieve having an initial Constraint Indexof about 1-12 which has been modified by being (i) pre-selectivated bycontacting with a first silicon source selected from the groupconsisting of silicones, tetraalkoxy silanes and organoamine silanepolymers, calcined, and (ii) treated with steam at about 280° C. about400° C.; so that para-substituted aromatic product selectivity of thecatalyst is increased over meta- and ortho-substituted aromatic product.12. The catalyst of claim 11 wherein the pre-selectivated, calcined,steam treated molecular sieve is subsequently contacted with a mixturecomprising toluene and a second silicon source which is a paraxyleneselectivation agent at reaction conditions for converting toluene toxylene for at least one hour to yield a twice modified catalyst.